mac80211: probe the AP when resuming

Check the connection by probing the AP (either using nullfunc or a
probe request). If nullfunc probing is supported and the assoc is no
longer valid, the AP will send a disassoc/deauth immediately.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>